feat(cynthia): add SSH CA

I am a Catgirl Authority OwO :3

Change-Id: Id1f2579fe20e3081ad895738211e78285de8d2f3
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2006
Reviewed-by: tazjin <mail@tazj.in>
Reviewed-by: cynthia <cynthia@tvl.fyi>
Tested-by: BuildkiteCI

users/cynthia/keys.nix

@@ -0,0 +1,7 @@
+{ ... }:
+
+{
+  all = [
+    "cert-authority ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAICsj3W6QczgxE3s5GGT8qg0aLrCM+QeRnSq9RkiZtKvz meow"
+  ];
+}